1. Field of the Invention
The present invention relates to a method and to a device provided for interferential measurement of the phase shift between two light beams coming from the same polarized source by fine measurement of the displacement of the fringes of an interference pattern between two light beams, one of the beams undergoing phase variations due to variations in the refractive index thereof, such as applied refractometry.
2. Description of the Prior Art
Among the various known refractometer types, there is the working with a monochromatic source whose beam is divided so as to pass concurrently through two tanks, one for the reference liquid and the other for the medium to be analysed, and with means for gathering the two beams together and for lighting a photodetector. Interferences, occuring because of the variation of the optical path resulting from the index variation and the intensity variations due to the displacement of the interference fringes, are measured.
French Patent 2,596,526 illustrates an example of refractometric detector in which each one of the (reference and measuring) tanks is independently part of an interferometry system, both tanks being supplied with light by the same source. Detection is achieved by two independent photodetectors which thus receive each a luminous intensity which sinusoidally varies as a function of the difference in the refractive indices between the reference tank or the measuring tank on the one hand and the air on the other. Individual calibration of each photometer is thus necessary. This optical system comprises a piezoelectric element designed to vibrate a mirror on which part of the light beam coming from the source reflects.
French Patent 2,697,336 describes a modulated phase type refractometer in which two beams that have passed respectively through a reference cell and a cell containing a medium whose refractive index undergoes variations are caused to interfere with each other. The two beams are formed from a single beam emitted by a laser after passing through a Pockels cell to which a fast alternating voltage suited to oscillate the interference pattern is applied, such as a sawtooth signal (FIG. 1) whose frequency is of the order of several KHz and whose amplitude is sufficient to obtain a displacement greater than 1.5 fringe (FIG. 2). The result on the interference pattern is the superposition of a slow variation linked with the composition variation of the medium and of a faster oscillating variation at the frequency of the alternating voltage. The intensity variations due to the fringe displacements are measured by a photometer and applied to electronics which converts the variations to logical signals and measurement of the phase shift between the cell control signal and the modulated intensity signals by counting by means of a clock signal. When the refractive index of the medium does not change, the output signal of the measuring phototransistor is that shown in FIG. 2.
The previous system eliminates many sources of inaccuracy of the prior systems insofar as the two interfering beams both come from the Pockels cell and therefore have the same optical &lt;&lt;past&gt;&gt;. However, for several reasons notably linked with the method used for measuring time lags between signal fronts, it appears that the accuracy that can be obtained (several %) can be considered insufficient for certain applications, notably in the field of chromatography.